The giant planet HAT-P-7b passes across the face of its star in this artist's concept. The planet, which is more massive than Jupiter, the giant of our own solar system, is so close to the star that the star heats its surface like a blowtorch. Scientists are studying the planet in detail to learn about the composition of its dense but super-heated atmosphere. [NASA/ESA/G. Bacon (STScI)]

Normally, a star heats its planets. The Sun shines on Earth, for example, and warms it up. But astronomers recently discovered a system where the thermostat is out of whack — a planet is cooling its star.

The planet in question is a so-called hot Jupiter — a gas giant so close in that its star heats it like a blowtorch. The star is called HAT P-7. It’s in the constellation Cygnus, the swan, which glides high across the sky on summer nights.

The planet is so massive and so close that its gravity pulls on the surface of HAT P-7 ever so slightly. That causes the star to puff out toward the planet a bit, lowering the surface temperature in that spot by just a third of a degree Fahrenheit. Although that doesn’t sound like much, it’s enough to create a small dark spot on the star’s surface, which astronomers detected with the Kepler space telescope.

Kepler was designed to find planets as they pass in front of their stars and dim their light. Because Kepler is above the distorting effects of Earth’s atmosphere, it can make extremely precise measurements of a star’s brightness.

Although Kepler didn’t discover the planet orbiting HAT P-7, it did observe the star more than a million times. Those observations revealed a tiny variation in brightness as the star rotated on its axis. And that’s how astronomers detected the stellar dark spot — the first time anyone has seen a planet induce such a change in temperature on the surface of a star.